I'm surprised no-one has reported this before, I guess not many people are using .NET 3.5. You can solve it by editing the page /Secure/Register.aspx and just remove the RepeatLayout="UnorderedList", it is only supported in .NET 4 or higher.
There will be a new release today and this will be fixed in the new release, but we will be dropping support for .NET 3.5 in the near future so my best recommendation is to use .NET 4 or 4.5 hosting and use the .NET 4 package of mojoPortal. I would not setup new sites under .NET 3.5.
Hope that helps,